US5233413A - Electronic image processing with automatic color correction - Google Patents

Electronic image processing with automatic color correction Download PDF

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US5233413A
US5233413A US07/639,826 US63982691A US5233413A US 5233413 A US5233413 A US 5233413A US 63982691 A US63982691 A US 63982691A US 5233413 A US5233413 A US 5233413A
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values
color
value
regions
luminance
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Hermann Fuchsberger
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Agfa Gevaert AG
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Agfa Gevaert AG
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/46—Colour picture communication systems
    • H04N1/56—Processing of colour picture signals
    • H04N1/60—Colour correction or control
    • H04N1/6027—Correction or control of colour gradation or colour contrast

Definitions

  • the invention relates generally to the preparation of an image of an original.
  • the invention relates to the automatic correction of color casts during electronic image processing.
  • an original is electrooptically scanned in the three primary colors at a multiplicity of points arranged in a series of columns and rows.
  • the resulting density signals are averaged over groups of the points to yield a reduced number of average density signals which represent an image of decreased resolution.
  • the average density signals are subsequently transformed into chrominance signals, i.e., color difference signals, and luminance signals in such a manner that a luminance signal and a pair of chrominance signals are associated with each group of points.
  • the modified signals are sent to a color exposure unit, e.g., a CRT scanner-printer system, which operates on points of the image serially.
  • the exposure unit converts the electrical signals into an optical image and recording material such as, for instance, color negative paper, is subsequently exposed to the image. It is noteworthy that the optical image is produced point-by-point through transformation of the electrical image signals.
  • Transformation into luminance and chrominance signals is known from the video art.
  • color corrections are performed by electronic means where there are aberrations in color, e.g., color casts, resulting from photography or where color falsification occurs during transfer and must be compensated for.
  • a color balance regulator is provided in practice and functions to effect a shift in color vector, for example, by increasing the amplification in one or two color channels. It is also possible to produce predetermined deviations from the standard gray point when the motif of the original makes this desirable, e.g., green lawns or red evening sky. Since an average is taken over the entire image, only overall compensation for or correction of color deviations or color casts can be achieved. Frequently, however, different color casts are present in areas of an image having different luminances.
  • Another object of the invention is to provide a method which makes it possible to more effectively compensate for color casts in an image.
  • An additional object of the invention is to provide a method which allows the coloration of an image to be improved in such a manner during electronic image processing that color casts of all types are automatically compensated for or eliminated throughout the image even when the luminance varies.
  • a further object of the invention is to provide an apparatus which enables the quality of an image to be improved.
  • Still another object of the invention is to provide an apparatus containing circuitry which allows the coloration of an image to be improved in such a manner during electronic image processing that color casts of all types are automatically compensated for or eliminated throughout the image even when the luminance varies.
  • One aspect of the invention resides in a method of preparing an image of an original.
  • the method involves scanning the original in each of the primary colors red, green and blue, and generating a luminance value for each of a multiplicity of regions of the original based on the scanning step.
  • the luminance values span a predetermined range and this range is divided into a plurality of segments so that different ones of the luminance values lie within different segments.
  • the regions of the original having luminance values within the segments are classified as color dominant or color nondominant.
  • At least one second value is calculated for each of selected segments and the calculations include assigning different weights to dominant and nondominant regions in such a manner that the second value for each selected segment represents an overall color cast for the regions of the respective segment.
  • Each of the second values is adjusted with reference to the gray point to generate a respective adjusted third value and a correction plot for each of the primary colors is established based on the pairs of second and third values.
  • the method of the invention permits automatic correction of color casts during electronic image processing.
  • the scanning step may be carried out electrooptically and scanning of the original may be performed at a multiplicity of points which define a series of N columns and M rows.
  • the generating step may then include generating red, green and blue density values for each point, dividing the points into groups each of which corresponds to one of the above regions of the original, and respectively averaging the red, green and blue density values for each of the groups to derive average red, green and blue density values for each region.
  • the points may be divided into a series of n columns and m rows per group where n is smaller than N and m is smaller than M.
  • the image represented by the average red, green and blue density values has lower resolution than the image represented by the initial red, green and blue density values.
  • the generating step may further comprise converting the average red, green and blue density values for each group into first and second chrominance values, i.e., color difference values, and a luminance value. Each segment of the luminance range containing a luminance value is then additionally associated with the two chrominance values corresponding to such luminance value.
  • the step of classifying various regions of the original as color dominants or color nondominants may include determining a regional color saturation value each region and employing a threshold value with each of the values. This preferably involves a comparison of each saturation value with a threshold value.
  • the step of calculating at least one second value for each selected segment may comprise generating first and second sums for each selected segment.
  • the first sum for each selected segment is obtained by adding the first chrominance values of the respective regions susceptible to color casts whereas the second sum is obtained by adding the second chrominance values of these regions.
  • the first and second sums may accordingly be considered to represent overall chrominance values for the different regions of the respective segments.
  • the step of adjusting the second values may be performed using the gray equivalence principle.
  • the adjusting step may involve shifting the first and second sums towards the gray point. If the first sum for the ith selected segment is represented by U i and the second sum by V i , this can be expressed mathematically as
  • the adjusting step may include shifting the first and second sums towards a predetermined color or color temperature which deviates from the gray point. This can be expressed mathematically as
  • V i + ⁇ V i V gi ( 4)
  • the shifted sums constitute adjusted overall chrominance values for the respective segments.
  • These adjusted overall chrominance values represent the original overall chrominance values as compensated for color casts.
  • the method may further comprise the step of converting the unadjusted overall chrominance values, i.e., the unshifted first and second sums, for each selected segment, together with the corresponding luminance value, into unshifted red, green and blue density values constituting unadjusted overall density values for the regions of the respective segment.
  • the adjusted overall chrominance values for each selected segment, together with the associated luminance value may be converted into shifted red, green and blue density values constituting adjusted overall density values for the regions of the corresponding segment.
  • the adjusted overall red, green and blue density values respectively represent the unadjusted red, green and blue density values as compensated for color casts.
  • Each unadjusted overall density value for a given primary color has a corresponding adjusted overall density value.
  • the points for each primary color are plotted to yield a correction plot for the corresponding color, preferably in a Cartesian coordinate system.
  • the plots may take the form of a series of connected straight line segments or of a monotonically increasing curve.
  • the data for constructing the correction plots may be stored and it is preferred to store the data for the different primary colors separately.
  • a discrete memory may be provided for each plot.
  • the memories may be erasable or reprogrammable and may, for instance, be in the form of look-up tables.
  • look-up table is intended to denote a commercially available electronic component which can store a series of values in the form of a table.
  • the plots may be employed to derive correction factors for the red, green and blue density values generated during point-by-point scanning of the original.
  • the red correction plot may be used to correct the red density values obtained upon point-by-point scanning of the original;
  • the green correction plot may be used to correct the green density values obtained upon point-by-point scanning of the original;
  • the blue correction plot may be used to correct the blue density values obtained upon point-by-point scanning of the original.
  • the segments of the predetermined luminance range may have identical spans. Alternatively, the spans of the segments may increase with increasing luminance.
  • the operation of assigning different weights to dominant and nondominant regions may involve the assignment of less weight to the chrominance values of dominant regions than to the chrominance values of nondominant regions.
  • the weight reductions may vary stepwise with degree of color dominance or can be based generally on distortion of the signals representing the chrominance values.
  • the dominant and nondominant regions are preferably distinguished from one another by determining regional color saturation values for the respective regions and comparing each saturation value with a threshold value.
  • a different threshold value may be associated with each segment of the luminance range and the threshold values may increase monotonically with increasing luminance.
  • These threshold values may be stored, e.g., in a look-up table.
  • the threshold values advantageously obey a linear relationship.
  • the dominant and nondominant regions are then distinguished from one another using the following criterium:
  • S ti denotes the threshold value for the ith segment
  • Y pi denotes the luminance value calculated for the ith segment
  • k 0 is a constant having a value between 10 and 20
  • k 1 is a constant having a value between 0.05 and 0.15.
  • the color cast correction factors calculated for each of the primary colors in the various segments of the predetermined luminance range may be plotted in the form of a series of connected straight line segments.
  • the method of the invention makes it possible to automatically correct not only global or overall color casts in the color channels associated with the original but also color casts which relate to a particular color and differ in different luminance intervals. Depending on the circumstances, three different correction plots may be obtained for the three primary colors red, green and blue.
  • the method according to the invention can be modified by scanning a plurality of originals, e.g., different originals constituting part of the same scene, and subsequently using all of the resulting density signals to establish respective correction plots for the three color channels. These correction plots are then applicable to each of the originals.
  • the apparatus comprises means for scanning the original in the three primary colors red, green and blue, and data processing means operatively associated with the scanning means.
  • the processing means includes means for generating a predetermined range of luminance values by assigning a respective luminance value to each of a multiplicity of regions of the original; means for dividing the predetermined range into a plurality of segments such that different ones of the luminance values lie in different segments; means for classifying the regions having luminance values within the segments as color dominants or color nondominants; means for calculating at least one second value for each of selected segments by assigning different weights to dominant and nondominant regions in such a manner that each second value represents an overall color cast for the regions of the respective segment; means for adjusting each second value with reference to the gray point so as to generate a respective adjusted third value; and means for establishing a correction plot for each of the primary colors from the pairs of second and third values.
  • the apparatus preferably further comprises respective storage means for the different correction plots.
  • the apparatus of the invention can be used with advantage for the automatic correction of color casts during electronic image processing.
  • FIG. 1 is a block diagram of an apparatus according to the invention for preparing an image of an original
  • FIG. 2 is a plot of one chrominance parameter versus another showing a series of curves of constant luminance
  • FIG. 3 is a plot of uncorrected color density versus corrected color density showing the development of a correction curve
  • FIG. 4 is a plot of uncorrected color density versus corrected color density showing three distinct correction curves for the primary colors red, green and blue, respectively.
  • FIG. 1 is a block diagram of an apparatus in accordance with the invention for preparing an image of an original.
  • the apparatus is designed for electronic image processing and is capable of automatically correcting for color casts during such processing.
  • a scanner 1 electrooptically scans an original 8 point-by-point.
  • the scanner 1 comprises a sensor in the form of a linear charge-coupled device, that is, a charge-coupled device having a series of sensing cells arranged in a row.
  • the charge-coupled device is moved vertically over the surface of the original 8 at constant velocity so that the original 8 is scanned at a multiplicity of image points defining a series of M columns and N rows.
  • the original 8 is scanned in each of the primary colors red, green and blue with scanning in these three primary colors being performed in succession. To this end, appropriate color filters are successively pivoted into the path of the light used for scanning.
  • the scanner 1 generates an electrical imaging signal for each point and each primary color. Every imaging signal represents the density of the corresponding point in the respective primary color.
  • the imaging signals obtained from the scanner 1, which are analog signals, are digitized in a non-illustrated analog-to-digital converter.
  • the resulting stream of digitized imaging signals is divided between a memory 2 which stores the digitized imaging signals and a data averaging unit 3 which is arranged in parallel with the memory 2.
  • the data averaging unit 3 functions to reduce the amount of data representing the electrooptical image derived from the scanner 1.
  • the data averaging unit 3 divides the N ⁇ M original image points, or the N ⁇ M pixels in each primary color, into a plurality of groups each of which corresponds to a predetermined region of the original 8.
  • the groups are formed in such a manner that the points or pixels of each group define a series of rows and columns.
  • Each row may contain k points or pixels and each column j points or pixels where j is smaller than N and k is smaller than M.
  • the data averaging unit 3 averages the density values for each primary color to yield respective average red, green and blue density values. Consequently, the original electrooptical image represented by the N ⁇ M original density values per primary color is replaced by an image of lower resolution constituted by n ⁇ m average density values per primary color.
  • the average density values are sent to a computer 4 which transforms the average red, green and blue density values for each group into a luminance value Y p and two chrominance values U p and V p according to the following equations:
  • R denotes average red density
  • G denotes average green density
  • B denotes average blue density
  • the luminance values obtained upon transformation span a predetermined luminance range.
  • This range is divided into a preselected number, i, of adjoining segments or intervals Y i , e.g., 32 segments or intervals.
  • i a preselected number
  • each group of points or pixels, and hence each predetermined region of the original 8 may be associated with a luminance interval Y i .
  • each set of luminance and chrominance values Y p ,U p ,V p is assigned to one of the luminance intervals Y i as follows:
  • the computer 4 now calculates the magnitude S pi of the color saturation vector for each group of points or pixels, i.e., for each predetermined region of the original 8, using the following equation:
  • the computer 4 next sorts or classifies each of the color saturation values S pi using a color saturation threshold value S ti .
  • the threshold value S ti used to differentiate between color dominant and color nondominant regions is not a constant but differs for the i luminance intervals.
  • the differentiating criteria S pi >S ti and S pi ⁇ S ti denote that the color saturation values S pi of the regions associated with the ith luminance interval are compared with the particular threshold value S ti for such interval.
  • the threshold values S ti are established empirically, increase with increasing luminance value.
  • U oi and V oi denote an average color cast for the respective luminance interval and are calculated before correcting for color casts.
  • the threshold values S ti for the individual luminance intervals are loaded into a look-up table and then retrieved by the computer 4 upon classification of the predetermined regions of the original 8 into color dominant and color nondominant regions.
  • the sensitometric properties of the color photographic recording materials used for reproduction of the corrected images are also considered during determination of the threshold values S ti .
  • a simplified method of calculating the threshold values S ti is based on the following linear relationship between S ti and luminance value Y:
  • k 0 is a constant having a value of 10 ⁇ k 0 ⁇ 20 and k 1 is a constant having a value of 0.05 ⁇ k 1 ⁇ 0.15.
  • the precise values of k 0 and k 1 within the indicated ranges is determined in dependence upon the sensitometric properties of the color photographic recording materials.
  • the threshold values S ti thus increase in direct proportion to the luminance.
  • FIG. 2 shows the U,V chrominance plane and illustrates a set of curves of constant luminance.
  • regions of identical color saturation having the same luminance value Y are located on circles whose radii represent the respective color saturation values.
  • the radii of the circles increase in direct proportion to the luminance value Y.
  • the color dominant regions for a given luminance value Y lie outside of the respective circle while the color nondominant regions which determine corrections in color casts are disposed inside the respective circle.
  • the luminance values Y pi for the respective luminance intervals remain unchanged upon correction or adjustment of the overall chrominance values U i ,V i .
  • R i ,G i ,B i respectively denote uncorrected red, green and blue density values derived from the uncorrected chrominance values U i ,V i while R i ',G i ', B i ' respectively denote corrected red, green and blue density values obtained from the corrected chrominance values U i + ⁇ U i ,V i + ⁇ V i .
  • Each uncorrected red density value R i is associated with a respective corrected red density value R i ' to define a point on a plot of R i versus R i '; each uncorrected green density G i is associated with a respective corrected green density value G i ' to define a point on a plot of G i versus G i '; and each uncorrected blue density value B i is associated with a respective corrected blue density value B i ' to define a point on a plot of B i versus B i '.
  • the original or uncorrected red density value R i (denoted by x), which corresponds to an overall color cast U i ,V i for the interval, is associated with a new, corrected red density value R i ' (see the points o).
  • R i red density value
  • This segmented plot can be transformed into a continuous curve by means of conventional mathematical smoothing algorithms. For instance, in one conventional method of smoothing a segmented plot, each point of the segmented plot is assigned a point P n on a plot representing an approximation of a final smooth curve and a new point is then established.
  • the new point is located midway between the original point P n and the center of the line connecting the points P n 31 1 and P n +1. This procedure can be repeated a number of times (iteration) until a smooth curve such as the inverted S-shaped curve of FIG. 3 is obtained.
  • a comparison of this line with the inverted S-shaped curve shows that R i ' ⁇ R i in the upper portion of the curve corresponding to high luminance of the image.
  • a correction curve is obtained for each of the three primary colors red, green and blue.
  • Each of the correction curves increases monotonically with increasing luminance value.
  • the red, green and blue density signals generated by the scanner 1 are transmitted along respective red, green and blue color channels to a printer 7 which issues a reproduction of the electrooptical image derived from the scanner 1.
  • the density signals are processed and such processing includes correcting the density signals for color casts.
  • the corrections are carried out by loading the data for each correction curve into an individual look-up table 5 which is associated with the respective color channel and is located in the path of the corresponding density signals.
  • the red, green and blue density signals generated by the scanner 1 are digitized and the resulting stream of digitized signals is divided between the memory 2 and the data averaging unit 3.
  • the digitized red, green and blue density signals leave the memory 2 along the respective red, green and blue color channels and are forwarded to the corresponding look-up tables 5.
  • the digitized density signals are modified by the look-up tables 5 in the manner outlined above with the density signals for each primary color being modified separately from the density signals for the other primary colors.
  • the look-up table 5 associated with the red color channel modifies the red density signals
  • the look-up table 5 associated with the green color channel modifies the green density signals
  • the look-up table 5 associated with the blue color channel modifies the blue density signals.
  • the modified density signals are transmitted to a memory 6 for temporary storage.
  • the modified density signals which represent the electrooptical image obtained from the scanner 1 as corrected for color casts, are retrieved from the memory 6 by the printer 7 which then records the image, e.g., on color paper.
  • the procedure outlined above can be used for the automatic correction of color casts.
  • This procedure is capable of eliminating not only global or overall color casts but, to the extent that the individual luminance intervals are associated with appropriate regions of the original 8, i.e., to the extent that the original 8 contains regions which are nearly gray, the previously mentioned "color imbalances" as well. If there are no nearly gray regions associated with a particular luminance interval, then no correction is performed in such interval. However, due to the smoothing of the correction plots described above, a correction is carried out in the next highest luminance interval. In contrast, during global or overall correction of color casts according to the prior art, color correction is performed for all intervals even when there are intervals which are not associated with nearly gray regions.

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Color Image Communication Systems (AREA)
  • Facsimile Image Signal Circuits (AREA)
  • Image Processing (AREA)
  • Processing Of Color Television Signals (AREA)
US07/639,826 1990-01-26 1991-01-09 Electronic image processing with automatic color correction Expired - Fee Related US5233413A (en)

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DE4002298 1990-01-26
DE4002298A DE4002298C2 (de) 1990-01-26 1990-01-26 Verfahren und Vorrichtung zur automatischen Korrektur von Farbstichen bei der elektronischen Bildverarbeitung

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JP (1) JP2588640B2 (enrdf_load_stackoverflow)
CH (1) CH681658A5 (enrdf_load_stackoverflow)
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JPH04213274A (ja) 1992-08-04
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DE4002298A1 (de) 1991-08-01
JP2588640B2 (ja) 1997-03-05

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